As an electronic display device of a light emitting type, an electroluminescence display (which is hereafter abbreviated as ELD) is known. Examples of constitutional elements of the ELD include inorganic electroluminescent elements (hereafter, referred to as inorganic EL elements) and organic electroluminescence elements (hereafter, referred to as organic EL elements).
Although such an inorganic EL element has been used as a flat type light source, in order to drive light emitting elements, a high voltage of alternating current is required. On the other hand, such as organic electroluminescence element has a constitution in which a light emitting layer attaining a compound capable of emitting light is sandwiched between a cathode and an anode. The organic electroluminescence element injects electrons and positive holes into the light emitting layer, recombines them so to produce exciters (exciton), and emits light by utilizing emission of light (fluorescence and phosphorescence) when the exciters are deactivated. Accordingly, the organic electroluminescence element can emit light with a voltage of about several volts to several ten volts. Further, since the organic electroluminescence element is a self-light-emission type, it has a rich view angle and a high visibility. Furthermore, since the organic electroluminescence element is a tin film type perfect solid state element, it is drawn to attention from the viewpoint of space-saving, portability, and the like.
Moreover, different from the primary light sources conventionally provided to practical use, such as light emitting diodes and cold cathode tubes, the organic electroluminescence element is a surface light source which is also a large feature. Examples of applications to which the above characteristics can be effectively utilized include illumination light sources and backlight for various displays. In recent years, especially, it is also suitable to use the organic electroluminescence element as backlight for liquid crystal full color displays for which demand increases remarkably.
In the case where such an organic electroluminescence element is used as such an illumination light source or backlight of a display, it is expected to be used as a light source to provide white color or so-called electric bulb color (hereafter, collectively called as white color). Examples of techniques to obtain white light emission with an organic electroluminescence element include the following methods. According to one of the methods, a plurality of light emitting materials different in wavelength of emission light is arranged in one element so as to obtain white color by mixture of colors. According to another one of the methods, multi color light emitting elements are coated with respective different colors of, for example, blue, green and red, and made to emit respective light simultaneously so as to obtain white color by mixture of colors. According to still another one of the methods, white color is obtained by use of color converting elements (for example, a combination of blue light emitting material and color converting fluorescence pigment).
With the judgment based on various request required for illumination light source and backlight, such as low cost, high productivity, and simple driving method, the method of arranging a plurality of light emitting materials different in wavelength of emission, light in one element and obtaining white color by mixture of colors may be preferable for these applications. In recent years, research and development have been made actively for the method.
The above-mentioned method of obtaining white color light will be described further in detail. That is, examples of the method include a method of employing two color light emitting elements having a relationship of complementary colors to each other in one element, such as blue light emitting material and yellow light emitting material and mixing colors so as to obtain white color; and a method of employing three color emitting materials of blue, green and red and mixing colors so as to obtain white color.
For example, according to the disclosed method (for example, refer to Patent Document 1 and Patent Document 2), three color phosphors of blue, green and red with high efficiency are doped as light emitting materials so as to obtain a white organic electroluminescence element. However, in the disclosed method, there is not only a problem that sufficient light emitting efficiency is not obtained, but also a problem that emission light color is fluctuated greatly due to change of current density.
In recent years, for the phosphor materials, phosphorescence luminescent materials are developed energetically so as to obtain an organic electroluminescence element with higher luminance (for example, Patent document 4 and Non-patent Documents 1 and 2). Light emission from the conventional phosphor materials is light emission from singlet excitation, and production ratio of singlet exciter to triplet exciter is (1:3). Accordingly, although the production probability of luminescence exciter species is 25%, since the upper limit of internal quantum efficiency becomes 100% due to exciter production ratio and internal conversion from the singlet exciter to the triplet exciter, the light emission efficiency becomes theoretically at most four times as compared with the case of fluorescence luminescent material. However, in particular, with regard to the phosphorescence luminescent material which provides blue light emission, there is no material which is excellent in light emission efficiency, disability, and especially, driving service life. In the current state, hitherto, it is difficult to develop a practical organic electroluminescence element which employs a blue phosphorescence luminescent material.
Moreover, reduction or elimination of barrier for charge injection between a cathode and an organic layer in order to attain low voltage and high brightness contributes greatly to improve the efficiency of a device. It is generally known to use metals having low work function such as alkali metals and alkaline earth metals in a cathode material. However, there are problems that these metals quickly diffuse in the neighboring organic layers, and cause deterioration of preservation stability under high temperature. Therefore, studies have been made to catch these alkali metals so as to prolong service life of the organic electroluminescence elements and to improve light emitting characteristics (for example, refer to Patent Document 3 and Patent document 4). However, there is no suggestion with regard to improvement in color fluctuation of light emission color.
Therefore, in the current state of the organic electroluminescence elements which are used as an illumination light source or backlight of displays and provide white light emission with a low driving voltage and high efficiency, an organic electroluminescence element is not realized such that the element is excellent in resistant characteristic for lowering of brightness due to continuous operation and preservation stability under high temperature and has less chromaticity fluctuation of light emission.